I cannot clock the ASP880 from an external clock source and/or I am experiencing clicks & pops.
Double check that you have set the clock source to external by using the front panel SAMPLE RATE switch, pressing it until it is flashing green. This selects external clock mode.
From here, double check your master clock source, and ensure that it is connected via a 75Ω BNC coaxial cable to the ASP880 BNC word clock input.
Providing that you have a valid clock source, the ASP880 should sync to it without issue and the SAMPLE RATE led in the switch will turn to solid illumination. This shows that the unit is locked. If you experience pops & clicks, double check any master/slave device configurations and cabling. A system should only have one master clock.
Here are some images with various products that may help to show you how to set up clocking correctly: